669. Trim a Binary Search Tree

Given a binary search tree and the lowest and highest boundaries as L and R, trim the tree so that all its elements lies in [L, R] (R >= L). You might need to change the root of the tree, so the result should return the new root of the trimmed binary search tree.

Example 1:

Input: 
    1
   / \
  0   2

  L = 1
  R = 2

Output: 
    1
      \

Example 2:
Input:

\
  0   4
   \

思路：
考虑到二叉搜索树的特性，根的值肯定大于左结点的值，肯定大于右结点的值。可以利用递归的思想。
判断root的值是否在[L,R]范围内
root.val小于L的值，则直接舍弃左子树，返回继续修剪以root.right为根结点的右子树；
root.val大于R的值，则直接舍弃右子树，返回继续修剪以root.left为根结点的左子树；
在范围内，则继续修剪左子树和右子树，然后返回root.

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public TreeNode trimBST(TreeNode root, int L, int R) {
        if(root == null)
            return root;
        if(root.val < L){
            return trimBST(root.right,L,R);
        }
        else if(root.val > R){
            return trimBST(root.left,L,R);
        }
        else{
            root.left = trimBST(root.left,L,R);
            root.right = trimBST(root.right,L,R);
            return